gc: restore stock-sized allocation pacing

Author: Frityet

notes/m6.md

@@ -843,3 +843,15 @@ M6 token-owned JIT controls:
   token across `jit.opt.start()`, then verifies the public call waits, publishes
   option flag/parameter changes, and leaves the token free after a later invalid
   option is caught.
+
+M6 GC2 stock-pacing compatibility:
+- Lowered `LJ_GC2_TRIGGER_MIN` from a 256 MiB floor to 256 KiB. The old floor
+  kept `collectgarbage("count")` growing through stock-style single-thread
+  `ffi.new()` bursts without an automatic step, failing the stock
+  `lib/ffi/ffi_new.lua` "GC step missing for ffi.new" check. The new floor
+  still batches allocation accounting above `LJ_GC2_ACCT_FLUSH`, but lets the
+  normal idle threshold bridge trigger cycles at LuaJIT-like heap growth.
+- `tests/t-ffi-cdata-alloc.lua` now includes the stock aligned-`ffi.new`
+  GC-step guard before the existing threaded cdata allocation stress.
+  Verification: direct stock `ffi_new.lua`, `m6_jit_alloc_account`, and
+  `m7_ffi_cdata_alloc` passed.

plan/05_gc_concurrent.md

@@ -619,6 +619,10 @@ driver ownership.
 ## 5.11 Pacing & assists
 Trigger when `alloc_since_trigger > trigger_bytes` where trigger_bytes =
 live_estimate * gcpause_pct/100 (default 100% ⇒ 2x heap growth, Lua-like).
+Current bridge: `trigger_bytes` keeps only a 256 KiB minimum floor
+(`8 * LJ_GC2_ACCT_FLUSH`). This preserves stock single-thread pacing for
+short bursts such as aligned `ffi.new()` loops while still batching GC2
+allocation-trigger accounting above the per-TG flush quantum.
 Hard limit `hard_bytes = trigger*2`: an allocating thread past it runs a
 bounded **mark assist** in alloc_slow: pop ≤2^assist_shift objects from the
 SSB-stack/steal and trace them (mutator tracing reuses worker code with

src/lj_gc2.h

@@ -143,7 +143,7 @@ enum {
 #define LJ_GC2_HS_STOPREQ		0x00000400u
 
 #define LJ_GC2_ACCT_FLUSH		32768u
-#define LJ_GC2_TRIGGER_MIN		(LJ_GC2_ACCT_FLUSH * 8192u)
+#define LJ_GC2_TRIGGER_MIN		(LJ_GC2_ACCT_FLUSH * 8u)
 #define LJ_GC2_HELPER_IDLE_STEP		(LJ_GC2_ACCT_FLUSH * 4u)
 #define LJ_GC2_WORKER_DRAIN_BATCH	64u
 #define LJ_GC2_WEAK_DRAIN_BATCH		64u

tests/t-ffi-cdata-alloc.lua

@@ -1,5 +1,6 @@
 local th = require"threading"
 local ffi = require"ffi"
+local bit = require"bit"
 local harness = require"thread_harness"
 
 local nthreads = harness.arg_number(1, "LJ_M7_FFI_CDATA_THREADS", 6)
@@ -10,6 +11,24 @@ typedef struct { int x; double y; } lj_m7_cdata_alloc_t;
 ]]
 
 assert(ffi.sizeof("lj_m7_cdata_alloc_t") == 16)
+
+do
+  local oc = collectgarbage("count")
+  for al = 0, 15 do
+    local align = 2 ^ al
+    local ct = ffi.typeof("struct { char __attribute__((aligned("..
+                          align.."))) a; }")
+    for _ = 1, 100 do
+      local cd = ct()
+      local addr = tonumber(ffi.cast("intptr_t", ffi.cast("void *", cd)))
+      assert(bit.band(addr, align - 1) == 0)
+    end
+  end
+  local nc = collectgarbage("count")
+  assert(nc < oc + 3000, "GC step missing for ffi.new")
+  collectgarbage("collect")
+end
+
 local ready, start = harness.channels(nthreads)
 local workers = {}